One problem related to coolant leaking has been reported for the 1999 Chevrolet Silverado 1500. The most recently reported issues are listed below.
I have a 1999 Chevrolet silverado truck with the 5. 3 liter v8. There has been an on going gradual loss of coolant with no apparent leak to be found. Upon further investigation the cause is due to the poor casting of the cylinder heads causing pressurized coolant to leak into the engine. The solution is to replace the castech (mfg) heads with new heads $1,100 each). Chevrolet is apparently aware of this common problem and has issued I believe service document 1986750. If gm is aware of this problem caused by the mis-manufacturing of the part a recall should be issued to all affected vehicles.